GEOCHEMISTRY AND MINERALOGY OF SULFIDE-RICH TAILINGS CHARACTERIZED BY CIRCUMNEUTRAL PORE WATER AND DRAINAGE

Geochemical and mineralogical investigation of the tailings deposit at the Greens Creek mine near Juneau, Alaska was performed to evaluate mechanisms controlling pore-water and drainage chemistry. This underground Ag-Zn-Pb-Au mine extracts sphalerite, tetrahedrite, galena, electrum and pyrargyrite from a massive sulfide deposit. The ore body is hosted in Triassic metasedimentary rocks primarily consisting of calcareous argillite and siliceous phyllite. The gangue mineral assemblage is dominated by pyrite, dolomite, and quartz. Calcite and barite also are common, whereas arsenopyrite and chalcopyrite are occasionally observed. Tailings core samples were collected from five boreholes ranging 7 to 26 m in total depth. Mineralogical investigation indicated that the occurrence of sulfide minerals follows the general order: pyrite >> sphalerite > galena, tetrahedrite > arsenopyrite and chalcopyrite. Pyrite generally accounted for 20 to 35 wt. % of tailings solids, whereas dolomite comprised ≤ 30 wt. % and calcite comprised 3 wt. % of tailings solids. The solid-phase geochemistry generally reflects the mineral assemblage; however, the presence of trace elements, such as Cd, Cr, Co, Hg, Mn, Mo, Ni, Se, Tl, and V, is attributed to elemental substitution into sulfide phases. Water samples were collected from tension lysimeters, monitoring wells and basal drains at various locations throughout the tailings facility. Pore-water pH generally ranged from 6.5 to 8.5, and the concentrations of SO₄, S₂O₃, Zn, Fe, Mn, Ni, As, Sb and Tl were variable, reflecting variations in redox conditions. Elevated concentrations of S₂O₃ and DOC in recently-placed tailings were attributed to residual process water and surface-bound processing compounds, and these constituents likely have an important influence on water chemistry. Iron and As were mobile under reduced conditions, whereas (thio)sulfate reduction and/or S₂O₃ disproportionation supported metal-sulfide precipitation and decreases in aqueous concentrations of metals and metalloids including Zn, Mn, Ni, Sb and Tl.